An RF amplifier used for broadcasting and communications is expected to amplify an RF signal at high efficiency and linearity. As for the amplifier, however, increasing the efficiency and increasing the linearity are incompatible in general. The efficiency of the amplifier exhibits such a characteristic as increasing with the power level of an input signal, and reaching the maximum efficiency in the neighborhood where the amplifier saturates. When using as the input signal a modulating wave with a large PAPR (Peak to Average Power Ratio) that has been used by broadcasting and mobile communications recently, the linearity deteriorates greatly at an operating point near the saturating point because of the clipping of a signal waveform due to saturation of the amplifier.
Therefore the RF amplifier used for broadcasting and communications is generally used at an operating level having a large output back-off from the saturating point. Accordingly, achieving the high efficiency at the operating level having the large output back-off from the saturating point is important. In contrast with this, as an effective technique of increasing the efficiency at the operating level having the large output back-off from the saturating point, a Doherty amplifier is reported.
For example, FIG. 14 shows a configuration of a Doherty amplifier as a conventional high efficiency amplifier described in Non-Patent Document 1, electrical lengths of its various portions, and impedances viewed from its various portions when the input signal level is small. The Doherty amplifier shown in FIG. 14 has an input terminal 1, an input splitting circuit 2, a class A or class AB biased carrier amplifier 3, an offset phase line 4, a 90° phase line 5, a phase line 6, a class B or class C biased peak amplifier 7, an offset phase line 8, a 90° phase line 9 and an output terminal 10.
In addition, FIG. 14 shows an impedance reference point 11 at the output side of the carrier amplifier 3, an impedance reference point 12 at the output side of the peak amplifier 7, and an output combining point 13 of the paths into which the input splitting circuit 2 splits the input. Here, the impedance reference point 11 at the output side of the carrier amplifier 3 is the point at which the load impedance seen by looking into the load side from the output side of the carrier amplifier 3 becomes maximum. Likewise, the impedance reference point 12 at the output side of the peak amplifier 7 is the point at which the impedance seen by looking into the output side of the offset phase line 8 from the output side of the peak amplifier 7 becomes maximum.
FIG. 15 shows the configuration of the Doherty amplifier as the conventional high efficiency amplifier described in the foregoing Non-Patent Document 1, the electrical lengths of its various portions, and the impedances seen from various portions when the input signal level is large. In FIG. 15, the same reference numerals as those of FIG. 14 designate the same components.
The offset phase line 4 connected to the carrier amplifier 3 has such an electrical length θc that will maximize the output impedance seen by looking into the output side of the carrier amplifier 3 from the impedance reference point 11 at the output side of the carrier amplifier 3. Likewise, the offset phase line 8 connected to the peak amplifier 7 has such an electrical length θp that will maximize the output impedance seen by looking into the output side of the peak amplifier 7 from the impedance reference point 12 at the output side of the peak amplifier 7. In addition, the electrical length of the 90° phase line 5 and that of the 90° phase line 9 are 90°, and the electrical length of the phase line 6 is 90+θc−θp.
The RF signal input via the input terminal 1 is divided by the input side splitting circuit 2 into two parts: a carrier amplifier 3 side path and a peak amplifier 7 side path. Along the carrier amplifier 3 side path, the RF signal from the input side splitting circuit 3 is supplied to the carrier amplifier 3, and the RF signal from the carrier amplifier 3 is supplied to the output combining point 13 via the offset phase line 4 and 90° phase line 5. On the other hand, along the peak amplifier 7 side path, the RF signal from the input side splitting circuit 2 is supplied to the peak amplifier 7 via the phase line 6, and the RF signal from the peak amplifier 7 is supplied to the output combining point 13 via the offset phase line 8. The output combining point 13 combines the RF signal from the carrier amplifier 3 with the RF output signal from the peak amplifier 7, and outputs the combined signal.
When the level of the input signal is small in FIG. 14, the class B or class C biased peak amplifier 7 becomes an OFF state, that is, the state in which the RF signal is not amplified. Thus, the output impedance of the peak amplifier 7 seen from the impedance reference point 12 at the output side of the peak amplifier 7 is ideally infinity (open) because of the effect of the offset phase line 8. In the conventional Doherty amplifier, the impedance reference point 12 and the output combining point 13 are connected directly so that they are considered to be the same point. Accordingly, the output impedance seen by looking into the peak amplifier 7 side from the output combining point 13 is ideally infinity (open).
In this case, assume that the load impedance seen by looking into the 90° phase line 9 from the output combining point 13 is R/2 (where R is the load resistance of the Doherty amplifier) and that the characteristic impedance of the 90° phase line 5 is R. Then, according to the impedance conversion effect by the 90° phase line 5, the load impedance seen by looking into the output side from the impedance reference point 11 at the output side of the carrier amplifier 3 becomes 2R, and only the RF signal from the carrier amplifier 3 is output from the output combining point 13.
On the other hand, when the input signal level is large in FIG. 15, the class B or class C biased peak amplifier 7 is brought into an ON state, that is, into the state in which the RF signal is amplified. Accordingly, at the output combining point 13, the RF signals from the carrier amplifier 3 and peak amplifier 7 are combined to be output. In this case, the load impedances seen by looking into the output side from the impedance reference point 11 at the output side of the carrier amplifier 3 and from the impedance reference point 12 at the output side of the peak amplifier 7 become R each.
Here, if the Doherty amplifier is designed in advance in such a manner that when the load impedance is 2R, the carrier amplifier 3 has low saturation power but high efficiency, and that when the load impedance is R, the carrier amplifier 3 and peak amplifier 7 each have large saturation power, then it is possible for the carrier amplifier 3 to operate at high efficiency when the input signal level is small, and for the carrier amplifier 3 and peak amplifier 7 to operate in such a manner that they have large saturation power when the input signal level is large.
According to the two functions, that is, the function that the output of the peak amplifier 7 is combined with that of the carrier amplifier 3 in response to the input signal level, and the function that the load impedance seen by looking into the output side from the carrier amplifier 3 and peak amplifier 7 varies in response to the input signal level, it becomes possible to implement the high efficiency operation in the state in which the output back-off from the saturation is large.
FIG. 16 illustrates efficiency characteristics versus output power of the Doherty amplifier. It is possible for the ideal Doherty amplifier to have two points at which the efficiency is maximum: the saturating point a of the Doherty amplifier; and the point b at which the output back-off is 6 dB as shown in FIG. 16. In FIG. 16, b is the first efficiency maximum point when only the carrier amplifier 3 operates when the input signal level is small, and a is the second efficiency maximum point when the carrier amplifier 3 and peak amplifier 7 operate when the input signal level is large.
Non-Patent Document 1: Youngoo Yang, Jeonghyeon Cha, Bumjae Shin, Bumman Kim, “A Fully Matched N-Way Doherty Amplifier With Optimized Linearity”, IEEE Trans. Microwave Theory Tech., vol. 3, pp. 986-993, March 2003.
In the Doherty amplifier as a conventional high efficiency amplifier, by using the 90° phase line 5 at the output side of the carrier amplifier 3, such conversion is implemented that the load impedance seen by looking into the output side from the impedance reference point 11 at the output side of the carrier amplifier 3 becomes 2R at a small signal and becomes R at a large signal. Therefore it is possible for the ideal Doherty amplifier to have two points at which the efficiency is maximum, that is, the saturating point of the Doherty amplifier and the point at which the output back-off is 6 dB. Conversely, it is theoretically impossible for the conventional Doherty amplifier to have the efficiency maximum point at an operating point at which the output back-off is greater than 6 dB. Thus, it has a problem of having its limit on achieving the high efficiency in a small signal region in which the output back-off is greater than 6 dB.
The present invention is implemented to solve the foregoing problem. Therefore it is an object of the present invention to provide a high efficiency amplifier capable of increasing the efficiency at the small signal operating level at which the output back-off is greater than 6 dB.